I want XM in my H2, but I am waiting for XM Direct to be released. This will be an "official" XM Radio product that will connected to and be controlled by GM radios.
The product should be out for GM applications in the July-August time frame.
Here's a link to XM Direct for BMWs to give you an idea of what the kit will look like:
http://www.xm411.com/phpbb/viewtopic.php?t=1804